The above tables need to be modified to meet existing installation conditions as per the following tables:

TABLE 310.15(B)(2)(A)

TABLE 310.15(B)(2)(b)

TABLE 310.15(B)(3)(a)

TABLE 310.15(B)(3)(c)

TABLE 310.15(B)(7)

These tables will be explained later.

1.1 Allowable Ampacities Tables Construction:

For example, Table 310.15(B)(16) which provides Allowable Ampacities of Insulated Conductors Rated Up to and Including 2000 Volts, 60°C Through 90°C (140°F Through 194°F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30°C (86°F).

Table 310.15(B)(16) will consists of:

1.1.A Conditions for
table application

These conditions are existing in the table description at the
top of the table (see below image), and in table 310.15(B)(16), these conditions are:

The right left one beside
copper conductors column is for copper conductors sizes,

The right one beside aluminum
or copper-clad aluminum conductors column is for aluminum or copper-clad
aluminum conductors sizes.

1.2 Notes for Allowable Ampacities Tables:

Note#1

The asterisk (*) next to the
ambient temperature of 30°C (86°F) included in table description at the top
of the table, Refer to table
310.15(B)(2) for application of the ampacity correction factors where the
ambient temperature is other than 30°C (86°F).

Note#2

The asterisk (**) next to
wire sizes 10, 12, and 14 AWG refers to article 240.4(D) and is letting you
know that conductor sizes #14, #12, and #10 copper and aluminum are limited
to certain size overcurrent protection devices (OCPD) even if the ampacity of
the #14, #12, and #10 conductor is higher. The limitations are:

A- For Copper Conductors:

#14 copper limited to 15 amp
OCPD

#12 copper limited to 20 amp
OCPD

#10 copper limited to 30 amp
OCPD

B- For Aluminum Conductors:

#14 aluminum not allowed

#12 aluminum limited to 15
amp OCPD

#10 aluminum limited to 25
amp OCPD

Note#3

Ampacity tables, particularly
Table 310.15(B)(16), do not take into account all the many factors affecting
ampacity.

If loads are not calculated
in accordance with the requirements of Article 220, the table ampacities,
even when corrected in accordance with ambient correction factors and the
notes to the tables, might be too high. Especially where many cables or
raceways are routed close to one another underground.

Note#4

Copper conductors of the same
size have three different allowable ampacities. The same is true for aluminum
(and copper-clad aluminum) conductors. This is because The maximum allowable
ampacities depend on the conductor’s temperature rating.

For example, a 3 AWG copper
conductor with a temperature rating of 60°C has a maximum allowable ampacity
of 85 amperes (A). The maximum allowable ampacity of the same 3 AWG copper
conductor with a temperature rating of 75°C is 100A. If the temperature
rating of the 3 AWG copper conductor is 90°C, the allowable ampacity is 115A
.

Step#1: Determine the proper table to use based on the existing wiring method conditions

For example, if we have a 6 AWG Type THWN copper wire in free air based on an ambient air temperature of 30° C (86° F), which table we will use?

Yes, it is table 310.15(B)(17), because tables Table 310.15(B)(16) and table 310.15(B)(18) don’t be used with single conductor in free air. Don’t use table TABLE 310.15(B)(19) because the ambient air temperature is 30° C, and THWN insulation is not listed in it.

Step#2: Determine the proper section in the selected table based on the conductor material type (Copper – Aluminum or Copper Clad Aluminum)

In same example above, because this is a copper conductor, we will use the section in the left side of table 310.15(B)(17).

Step#3: Locating the Ampacity Value

In the table section selected above, move down to the given conductor size, which is 6 AWG in our example, the amapcity value are existing in the 6 AWG Row.

Now, move to the right in the 6AWG row until you reach the column for THWN insulation for a copper conductor. This will be the 75° C (167° F) column.

Step#4: Reading the Ampacity Value

Now read the amapcity of the conductor, you should read 95.

Another example:

What is the allowable ampacity of a non-metallic-sheathed cable containing three 12 AWG Type THW-2 Copper wires based on an ambient air temperature of 30° C(86° F)?

Solution:

Step#1: Determine the proper table to use

Because this cable containing three conductors, use table Table 310.15(B)(16). Don’t use table 310.15(B)(17) or table TABLE 310.15(B)(19) because there are three conductor in a cable. Don’t use table 310.15(B)(18) because the ambient air temperature is 30° C, and THW-2 insulation is not listed in it.

Step#2: Determine the proper section in the selected table based on the conductor material type (Copper – Aluminum or Copper Clad Aluminum)

Because these are copper conductors, we will use the section in the left side of table Table 310.15(B)(16).

Step#3: Locating the Ampacity Value

In the table section selected above, move down to the given conductor size, which is 12 AWG , the amapcity value are existing in the 12 AWG Row.

Now, move to the right in the 6AWG row until you reach the column for THW-2 insulation for a copper conductor. This will be the 90° C (194° F) column.